Mineral oil lubricants are derived from various crude oil stocks by a variety of refining processes directed towards obtaining a lubricant base stock of suitable boiling point, viscosity, viscosity index (VI) and other characteristics. Generally, the base stock will be produced from the crude oil by distillation of the crude in atmospheric and vacuum distillation towers, followed by the separation of undesirable aromatic components and finally, by dewaxing and various finishing steps. Because aromatic components lead to high viscosity and extremely poor viscosity indices, the use of asphaltic type crudes is not preferred as the yield of acceptable lube stocks will be extremely low after the large quantities of aromatic components contained in the lubestocks from such crudes have been separated out; paraffinic and naphthenic crude stocks will therefore be preferred but aromatic separation procedures will still be necessary in order to remove undesirable aromatic components. In the case of the lubricant distillate fractions, generally referred to as the neutrals, e.g. heavy neutral, light neutral, etc., the aromatics will be extracted by solvent extraction using a solvent such as furfural, N-methyl-2-pyrrolidone, phenol or another material which is selective for the extraction of the aromatic components. If the lube stock is a residual lube stock, the asphaltenes will first be removed in a propane deasphalting step followed by solvent extraction of residual aromatics to produce a lube generally referred to as bright stock. In either case, however, a dewaxing step is normally necessary in order for the lubricant to have a satisfactorily low pour point and cloud point, so that it will not solidify or precipitate the less soluble paraffinic components under the influence of low temperatures.
A number of dewaxing processes are known in the petroleum refining industry and of these, solvent dewaxing with solvents such as methylethylketone (MEK), a mixture of MEK and toluene or liquid propane, has been the one which has achieved the widest use in the industry. Recently, however, catalytic dewaxing processes have entered use for the production of lubricating oil stocks and these processes possess a number of advantages over the conventional solvent dewaxing procedures. These catalytic dewaxing processes are generally similar to those which have been proposed for dewaxing the middle distillate fractions such as heating oils, jet fuels and kerosenes, of which a number have been disclosed in the literature, for example, in Oil and Gas Journal, Jan. 6, 1975, pp. 69-73 and U.S. Pat. Nos. RE 28,398, 3,956,102 and 4,100,056. Generally, these processes operate by selectively cracking the normal and slightly branched paraffins to produce lower molecular weight products which may then be removed by distillation from the higher boiling lube stock. A subsequent hydrotreating step may be used to stabilize the product by saturating lube boiling range olefins produced by the selective cracking which takes place during the dewaxing. Reference is made to U.S. Pat. Nos. 3,894,938 and 4,181,598 for descriptions of such processes.
The catalysts which have been proposed for these dewaxing processes have usually been zeolites which have a pore size which admits the straight chain, waxy n-paraffins either alone or with only slightly branched chain paraffins but which exclude more highly branched materials and cycloaliphatics. Intermediate pore size zeolites such as ZSM-5, ZSM-11, ZSM-12, ZSM-22, ZSM-23, ZSM-35, ZSM-38 and the synthetic ferrierites have been proposed for this purpose in dewaxing processes, as described in U.S. Pat. Nos. 3,700,585 (Re 28398); 3,894,938; 3,933,974; 4,176,050; 4,181,598; 4,222,855; 4,259,170; 4,229,282; 4,251,499; 4,343,692, and 4,247,388. A dewaxing process employing synthetic offretite is described in U.S. Pat. No. 4,259,174. Processes of this type have become commercially available as shown by the 1986 Refining Process Handbook, page 90, Hydrocarbon Processing, September 1986, which refers to the availability of the Mobil Lube Dewaxing Process (MLDW). The MLDW process is also described in Chen et al "Industrial Application of Shape-Selective Catalysis" Catal. Rev.-Sci. Eng. 28 (283), 185-264 (1986), especially pp. 241-247, to which reference is made for a further description of the process. Reference is made to these disclosures for a description of various catalytic dewaxing processes.
In the catalytic dewaxing processes of this kind, the catalyst becomes progressively deactivated as the dewaxing cycle progresses and to compensate for this, the temperature of the dewaxing reactor is progressively raised in order to meet the target pour point for the product. There is a limit, however, to which the temperature can be raised before the properties of the product, especially oxidation stability become unacceptable. For this reason, the catalytic dewaxing process is usually operated in cycles with the temperature being raised in the course of the cycle from a low start-of-cycle (SOC) value, typically about 500.degree. F. (about 260.degree. C.), to a final, end-of cycle (EOC) value, typically about 680.degree. F. (about 360.degree. C.), after which the catalyst is reactivated or regenerated for a new cycle. Typically, the catalyst may be reactivated by hydrogen stripping several times before an oxidative regeneration is necessary as described in U.S. Pat. Nos. 3,956,102; 4,247,388 and 4,508,836 to which reference is made for descriptions of such hydrogen reactivation procedures. Oxidative regeneration is described, for example, in U.S. Pat. Nos. 4,247,388; 3,069,363; 3,956,102 and G.B. Patent No. 1,148,545. It is believed that the hydrogen reactivation procedure occurs by transfer of hydrogen to the coke on the deactivated catalyst to form more volatile species which are then stripped off at the temperatures used in the process.
The use of a metal hydrogenation component on the dewaxing catalyst has been described as a highly desirable expedient, both for obtaining extended dewaxing cycle durations and for improving the reactivation procedure even though the dewaxing reaction itself is not one which required hydrogen for stoichiometric balance. U.S. Pat. No. 4,683,052 discloses the use of noble metal components e.g. Pt, Pd as superior to base metals such as nickel for this purpose. During the dewaxing cycle itself, nickel on the catalyst was thought to reduce the extent of coke lay-down by promoting transfer of hydrogen to coke precursors formed on the catalyst during the dewaxing reactions. Similarly, the metal was also thought to promote removal of coke and coke precursors during hydrogen reactivation by promoting hydrogen transfer to these species to form materials which would be more readily desorbed from the catalyst. Thus, the presence of a metal component was considered necessary for extended cycle life, especially after hydrogen reactivation.